The present invention relates to an image sensor; and, more particularly, to an image sensor incorporating therein a capacitor structure for improving an optical efficiency of the image sensor.
As is well known, an image sensor is a semiconductor device for sensing a light reflected from an object to generate an image data. Especially, an image sensor fabricated by using a complementary metal oxide semiconductor (CMOS) technology is called a CMOS image sensor.
Generally, the CMOS image sensor includes a plurality of unit pixels. Each of the unit pixels also includes a light sensing element and a plurality of transistors. The light-sensing element such as a photodiode senses incident light reflected from an object and accumulates photoelectric charges that are generated due to the incident light. The transistors control a transfer of the photoelectric charges.
In FIG. 1, there is shown a prior art image sensor 100 including: a silicon substrate 102, a photodiode 120 formed into the silicon substrate 102 for sensing a light beam incident thereto and generating photoelectric charges, a capacitor structure 130 formed on the silicon substrate 102 and the photodiode 120, a diffusion region 106, a transfer transistor 110 formed on the photodiode 120, the diffusion region 106 and an isolation region 104.
In the image sensor 100, the transfer transistor 110 is provided with a gate dielectric 112, a gate electrode 114 and a spacer 116. The transfer transistor 110 is coupled to a sensing node for transferring the photoelectric charges to the sensing rode in response to a transfer control signal. The capacitor structure 130 is provided with an insulating film 132, a bottom electrode 134, a capacitor dielectric 136 and a top electrode 138. In case when the photodiode 120 does not have sufficient capacitance, a certain portion of the photoelectric charges cannot be stored in the photodiode 120, which will, in turn, leak out into the silicon substrate 102, thereby decreasing an overall optical efficiency thereof and making noses in the image sensor 100. These problems can be eliminated by using the capacitor structure 130 which is capable of providing additional capacitances to the photodiode 120.
In FIGS. 2A to 2D, there are illustrated manufacturing steps involved in manufacturing the conventional image sensor 100.
The process for manufacturing the conventional image sensor 100 begins with the preparation of an active matrix having a silicon substrate 102, a transfer transistor structure 110 formed thereon, an isolation region 104 and a photodiode 120 formed into the silicon substrate 102, as shown in FIG. 2A. An insulating layer 132, e.g., made of SiOx, is formed over the entire surface by using a chemical vapor deposition (CVD) technique. The photodiode 120 is capable of converting a light beam incident thereto into photoelectric charges. The transfer transistor 110 includes a gate oxide 112, a gate electrode 114 and a spacer 116. The transfer transistor 110 is coupled to a sensing node 106 for transferring the photoelectric charges to the sensing node 106 in response to a transfer control signal. The sensing node 106 can be connected to a transistor such as a reset transistor or an amplification transistor not shown for the sake of simplicity.
Referring to FIG. 2B, an insulating layer 132, e.g., made of silicon oxide (SiO2), is formed on top of the active matrix by using a method such as CVD. Thereafter, the insulating layer 132 is patterned into a first predetermined configuration to form a contact hole.
In a next step, a bottom electrode layer 134, a capacitor dielectric layer 136 and a top electrode layer 138 are formed on the insulating layer 132, successively, as shown in FIG. 2C.
Finally, the top electrode layer 138, the capacitor dielectric layer 136 and the bottom electrode layer 134 are patterned into a second predetermined configuration, thereby obtaining a capacitor structure 130.
One of the major shortcomings of the above-described image sensor 100 is that it has complex manufacturing steps to form the capacitor structure 130 on the photodiode 120.
It is, therefore, an object of the present invention to provide an image sensor incorporating therein a capacitor structure for improving an optical efficiency thereof.
It is another object of the present invention to provide a method for manufacturing an image sensor incorporating therein a capacitor for improving an optical efficiency thereof.
In accordance with an aspect of the present invention, there is provided an image sensor provided with a plurality of unit pixels, each unit pixel comprising: a photoelectric element for sensing a light beam incident thereto and generating photoelectric charges; a transistor including a gate dielectric formed adjacent to the photoelectric element and a gate electrode formed on top of the gate dielectric; and a capacitor structure including an insulating film formed on a portion of the photoelectric element and a bottom electrode, wherein the insulating film and the gaze dielectric are made of a same material and the bottom electrode and the gate electrode are made of a same material.
In accordance with another aspect of the present invention, there is provided a method for manufacturing an image sensor, the method comprising the steps of: a) preparing a silicon substrate; b) forming a first dielectric layer and a first conductive layer, successively; c) patterning the first dielectric layer and the first conductive layer to obtain an insulating film and a bottom electrode of a capacitor structure and a gate dielectric and a gate electrode of a transistor, simultaneously; d) implanting a first type of dopants into a portion of the silicon substrate which is not covered with the insulating film and the gate dielectric and placed therebetween, thereby forming a photoelectric element; e) forming a second dielectric layer; f) removing a portion of the second dielectric layer which is located on top of the photoelectric element, thereby forming a contact hole; g) forming a second conductive layer on top of the second dielectric layer and the contact hole; and h) removing portions of the second conductive layer and the second dielectric layer which are placed on top of the gate electrode and the remaining portion of the photoelectric element.